For wireless power transmission or wireless energy transfer technologies for transferring electric energy to desired devices in a wireless manner, electric motors or transformers using the principle of electromagnetic induction began to be used in the 1800s. Since then, methods of emitting radio waves or electromagnetic waves such as laser to transfer electric energy have been attempted.
Electric toothbrushes and some wireless shavers that are commonly used are also charged using the principle of electromagnetic induction. Electromagnetic induction refers to a phenomenon in which a current flow by the voltage induced when a magnetic field is changed around a conductor.
Electromagnetic induction technology has been rapidly commercialized in association with small devices, but has a disadvantage in that power can only be transmitted short distance. To date, the only method of transferring energy in a wireless manner besides electromagnetic induction technology is a long-distance transmission technology using resonance and short-wave radio frequencies.
In recent years, among these wireless power transmission technologies, a method of transferring energy using resonance is frequently used. In wireless power transmission systems using resonance, electric power is transferred through transmission-side and reception-side coils in a wireless manner, and therefore a user can easily charge electronic devices such as portable devices.
In particular, an electronic device is placed on a wireless power transmission device to be charged in a wireless manner, and the wireless power transmission device conventionally transmits electric power through only a single transmission coil included therein. For this reason, since the position of a reception coil included in the electronic device is always different, power transmission efficiency may be lowered.
That is, when only the single transmission coil included in the wireless power transmission device, a region in which magnetic flux is cancelled out may be present depending on the position of the reception coil, in which case power transmission may not be smoothly performed.